Many enterprises are moving towards using VoIP over their LANs and WLANs to interconnect various terminal devices for voice communications. Moreover, external voice communications with remote parties through, for example, the PSTN, may be converted to VoIP communications for routing within the enterprise communication system. A Private Branch exchange (PBX) typically acts as the interface between the PSTN and the internal enterprise communication system. The PBX provides conversion between digital circuit-switched calls and VoIP calls, and assists in routing calls to the correct terminal device. SIP signaling is commonly used to set-up, manage, and tear-down media paths for the VoIP calls.
Conventional mobile devices are configured for cellular wireless communications. Cellular communication may take place using any one of a number of cellular protocols, including GSM, CDMA, etc. Now, many mobile devices are configured for both cellular communications and IP communications. These are referred to a “dual-mode” devices.
Dual-mode devices may include, for example, handheld mobile devices, but can also include other devices configured for both cellular and IP communications. For example a “softphone” implemented on a laptop with WiFi connectivity may be a dual-mode device.
VoIP calls are often preferred over cellular calls when a device is within IP network coverage limits (e.g. on a WLAN campus), since the WLAN owner/enterprise may charge little or nothing for the VoIP call, and the enterprise, if any, can often provide enhanced services and maintain better control over the session. Nevertheless, cellular typically offers much more extensive coverage than the limited range associated with a WLAN installation. Accordingly, a need may arise to switch an active VoIP call to a cellular network.
Similar reference numerals may have been used in different figures to denote similar components.